This invention generally relates to free electron lasers and more specifically to magnets used for producing free electron laser emission from relativistic electrons.
Nearly 30 years ago reports were made of spontaneous and stimulated emissions of electromagnetic radiation in a spatially periodic transverse magnetic field (i.e., undulator radiation). More recently, electrons at relativistic energies have been passed through spatially periodic transverse magnetic fields to obtain undulator emissions in a much broader range to include free electron emission at about 3 microns. Research is being carried out to develop free electron lasers for operation over a wide range of wavelengths from the far infrared (400 microns) through the visible and ultraviolet light spectra and into the X-ray wavelengths.
As this research has continued, investigations into the application for such lasers have also been undertaken. There are a wide variety of applications emerging, especially in the studies of solid state excitations. For example, laser light in the range from 10,000 microns to 1 micron can be used in the study of cyclotron resonance in narrow band and other semiconductors, and of plasmons and Mosfets. Superconductive lattice, interface, Mosfet 2-D states, shallow impurity states, phonons, and molecular rotational transitions for chemisorbed and absorbed surfaces are other areas of study where laser light can be utilized advantageously.
The major elements of a free electron laser include an accelerator for producing electrons at relativistic energies, an undulator magnet for producing the spatially periodic transverse magnetic field, and a beam transport system for directing the electrons, in a vacuum, from the accelerator through the undulator magnet. Equation 1 defines the peak wavelength of undulator radiation (.lambda.) where .tau..sub..mu. is the period of the undulator, .gamma. is related to the energy of the electron according to equation 2, P is the transverse momentum imparted to the electron by the undulator by the spatially periodic field, and B is the amplitude of the undulator field.
From the foregoing it is apparent that the electromagnet for producing the spatially periodic magnetic field must have several charcteristics. Periodicity of the magnet must be constant. The amplitude of the field also must be constant. Moreover, the magnet must have a relatively large bore to minimize optical cavity losses. However, increasing the bore causes the free electron laser gain to decrease. Moreover, the magnet motive force requirements must also be increased and this requires, an increase in the power to the electromagnet.
In the prior art a number of magnets have produced spatially periodic fields. In some, used especially at very short wavelengths, permanent magnets or discreet electromagnets are positioned along a bore in order to provide the spatially periodic fields. In other applications it has been proposed to make a field by winding a two-conductor coil in a helix. With this approach, equation 4 provides a reasonable approximation of the total current in the coil that is required to produce the field. There is no iron in this proposed magnet, and the term "a" defines the effective radius of the coil. In a practical application, the effective radius of the coil lies approximately at the midpoint of the coil taken in a radial direction. In one particular application where the cavity diameter was to be approximately 12 cm in the field and the axis was to be 500 gauss with a 20 cm undulator wavelength, the power requirement for producing the field was approximately 800 kw.
The addition of a helical iron pole with the helical coil and a return path of magnetic materials was also reviewed. Analysis of this structure showed a reduction of the power by a factor of 8. However, magnet would be complex to manufacture. Even at this lower power, conventional regulated dc power supplies were not readily available.